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Relays vs solid state

Discussion in 'Electronic Components' started by Steve Widmark, Dec 20, 2003.

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  1. What are the advantages of using a relay instead of an SCR or triac?

    Steve Wimark
    Mountain View High School, Mountain View, CA
     
  2. Relays are cheaper if a large amperage is involved. A common
    example is in automotive starting systems. When automotive electric
    systems go to 42 volts, only 1/3 the amps will be required so that
    transistors become more competitive.
     
  3. Chaos Master

    Chaos Master Guest

    Steve Widmark() said those words of arcane wisdom from the
    elves:
    For some applications (e.g. automotive) where high amperage is involved, relays
    are cheaper.
     
  4. Guest

    Others have already answered regarding relays being
    cheaper at high currents.

    In addition, an SCR or triac will put more resistance in the
    circuit than a relay will. Sometimes that higher resistance
    will prevent proper circuit operation, so a relay rather than
    and SCR or triac is needed. Also, that resistance will develop
    heat when current flows through it, and it will reduce the voltage.
    The amount of resistance is very small, but when high currents
    are involved, it becomes a significant factor. A relay will
    also put some resistance in the circuit - but FAR less than a
    triac or SCR, so the heat produced is way less, and the voltage
    reduction is insignificant.

    This gets a little more complicated:
    Relays function differently than SCR's. When you send
    a signal to turn an SCR on, it stays on until one of
    two things happens - either the circuit that the SCR
    is controlling stops drawing sufficient current to
    keep the SCR conducting, or a different signal is sent
    to the SCR to turn it off. On the other hand, a relay
    will stay on only as long as the signal telling it to turn
    on is present. (And as long as that signal is present, the
    relay will stay on, even if the circuit it is controlling
    is drawing no current at all.) If you want it to stay on,
    you either have to ensure that that relay's turn-on signal
    will stay on, or use a special kind of relay called a latching
    relay.

    A triac is similar to an SCR. However, a triac used in an
    AC circuit will reach one of the turn off conditions
    I mentioned for the SCR "automatically" since the
    circuit is AC. The circuit the triac controls will stop
    drawing current when the AC sine wave reaches 0 volts.
    Any of these differences could make one device more
    preferable than the other for the particular circuit.

    Now, a little more complexity:
    An inductive device such as a transformer, a motor, a fluorescent
    light etc. produces an "inductive kick" when it is turned
    off. Solid state devices can be burned out by these,
    and triacs can fail to turn off even when not damaged by them.
    So with SCR's and triacs controlling inductive devices,
    special design consideration has to be given to ensure
    both that the circuit will work and that it won't be damaged.
    A relay can also be damaged, but it is far less susceptible
    to damage form the inductive kick than an SCR or triac, all other
    conditions being equal. Where this shows up in the consumer
    world is in dimmers that are to be used for incandescent
    loads only. For example, if you want to dim low voltage halogen
    lights which get their power from a transformer, you can't use
    a regular dimmer. Some remote control systems such as X-10
    or Smarthome have "appliance modules" and "light modules".
    The "light modules" have a triac that turns the lights on
    or off (and sometimes dims them) while the "appliance modules"
    use a relay. When I added low voltage outdoor landscape
    lights to the circuit for my outdoor lighting, the remote
    (X-10) system could no longer turn the lights off! The low
    voltage system uses a transformer, and at turn-off the
    inductive kick it produces prevented the triac from turning
    off.
     
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